One of the simplest forms of microwave resonators having microstripline structures is a resonator that is formed with conductor striplines each having an electrical length of a half wavelength (180 degrees) or an electrical length integral-number times as large as a half wavelength in resonance frequency, and a dielectric substrate and a conductor ground plate. This resonator resonates in a mode in which currents flow along the striplines. In a resonant state, the current density distribution concentrates mostly at each edge portion of the striplines. This tendency becomes more remarkable as the frequency becomes higher.
In a case where a resonator of the above type is used as a microwave resonator for high-power signals of 1 W or greater, for example, the current concentration at each edge portion of the striplines hinders achievement of high power handling capability. This is because the high current density at the edge portions exceeds the allowable current density of the conductive material, and the electrical conducting properties of the conductive material are degraded. If the striplines are made of a superconducting material, for example, this phenomenon is observed, as the current density at the edge portions exceeds the critical current density of the superconducting material.
Along with the power handling capability, the Q value is another important element in the characteristics of a resonator. The Q value of a resonator represents the sharpness of the resonance peak on the frequency domain, and is determined by the resonator loss caused by various factors, such as conductor loss, dielectric loss, and radiation loss. When the loss is small, the Q value is high. In a frequency filter device such as a low-pass filter, a high-pass filter, or a bandpass filter that is formed with resonators, sharper cutoff characteristics and smaller insertion loss are achieved when the Q value of each resonators is higher. Therefore, resonators having high Q values are often demanded.
If the dominant loss factor that determines the Q value of a resonator of the above type is conductor loss, the current concentration at the edge portions of the striplines also becomes a problem. Due to the current concentration, the effective cross-sectional area of the striplines becomes smaller, and the resistance becomes higher. As a result, the conductor loss becomes greater, and the Q value becomes lower. There are cases where the electrical resistance at the current concentrating portion becomes higher, and the conductor loss becomes greater, resulting in a lower Q value.
As a technique for reducing the current concentration at the edge portions of the striplines, JP-A-H8-321706 (KOKAI) discloses a technique by which slits are formed at regular intervals along the lines in the entire linear stripline unit. As an improved version of this method, JP-A-H11-177310 (KOKAI) discloses a technique by which one or more slits are formed along the striplines only at the edge portions of the striplines.